1. Field of the Invention
The present invention relates to an optical fiber which is used as an optical component and a fiber type optical component using the optical fiber. The present invention particularly relates to a fiber type optical component made by enhancing the refractive index by emitting light from the side direction of the fiber.
2. Description of Related Art
Various optical components made by adding material having light sensitivity to optical fibers and emitting light according to the light sensitive material so as to alter the refractive index are produced for practical use. Here, the light sensitivity is a property in which the refractive index is altered by emission of light. As an example, an optical fiber grating may be mentioned. An optical fiber grating is made by adding a predetermined periodical perturbation to the area where the light of the optical fiber is transmitted. Usually, such a perturbation can be indicated by alteration of the refractive index. This periodic alteration of the refractive index is commonly obtained by adding the material having light sensitivity on the optical fiber and emitting light on the optical fiber according to the light sensitivity of the material. In order to obtain such a periodic refractive index, a holographic method, phase mask method, step-by-step method or the like are commonly employed.
The refractive index of the section where the light sensitive material is applied is enhanced by absorbing the emitted light. That is, the intensity of the emitted light decreases when the emitted light passes the area where the light sensitive material is added. Therefore, in above mentioned methods in which the light is emitted from the side of the optical fiber, the refractive index on the emitted side may easily increase, and the increase of refractive index is restricted on the opposite side; thus, there was a concern that uniform increase of the refractive index cannot be obtained in the cross section of the optical fiber. However, conventionally, the light sensitive material was applied only on the core section of the optical fiber, and the outer diameter of the core section is as small as 10 xcexcm at maximum. Thus, the non-uniformity of the increase of the refractive index was not regarded as problem.
However, recently, it is required to form a grating by using a fiber to cladding of which light sensitive material is applied like a grating for restricting the combination with the cladding mode in the Bragg grating and the slant Bragg grating. In the case of such a fiber grating, when the outer diameter of the light sensitive layer which is added to the cladding decreases, the required characteristics cannot obtained; therefore, the outer diameter of the area where the light sensitive material is applied must be increased to as much as 20 xcexcm or more.
In contrast, when the outer diameter of the light sensitive layer is increased and is too large, the increase of the refractive index near the core section may be restricted due to non-uniform increase of the refractive index and polarization dependence loss occurs in optical components. Thus, it was a problem that the influence caused by non-uniform refractive index cannot be ignored.
The present invention was made in consideration of such a situation. An object of the present invention is to obtain a fiber structure in which the required property is provided and the influence due to the non-uniform increase of the refractive index is minimized in the optical fiber to the cladding and the core of which the light sensitive material is applied. Also, it is an object of the present invention to provide the fiber type component using this optical fiber, particularly a grating type optical component.
In order to solve the abovementioned problems, the invention according to a first aspect is a fiber type optical component comprising a core, a cladding, a refractive index enhancing section, wherein the core and the cladding have light sensitivity, the refractive index enhancing section is formed by emitting ultraviolet light on an optical fiber, density of germanium in a layer having light sensitivity on the core and the cladding in the optical fiber satisfies formula 1 such as       24    ⁡          [                        µm          ·          wt                ⁢                  xe2x80x83                ⁢        %            ]         less than             ∑      k        ⁢                  β        k            ⁢              d        k               less than       100    ⁡          [                        µm          ·          wt                ⁢                  xe2x80x83                ⁢        %            ]      
and the amount of alteration of refractive index of the core and the cladding by the emission of ultraviolet light is less than 0.002 at of 674.9 nm wavelength. Here, in formula 1, xcex2k is density of GeO2 in each optical fiber layer, and the dk is thickness in the radius direction of each optical fiber layer.
The invention according to a second aspect is a fiber type optical component comprising, a core, a cladding, a refractive index enhancing section, wherein the core and the cladding have light sensitivity, the refractive index enhancing section is formed by emitting ultraviolet light on an optical fiber, a layer having light sensitivity on the core and the cladding in the optical fiber satisfies formula 2 such as   0.35   less than       exp    ⁢          (                        -          2                ⁢                              ∑            k                    ⁢                                    α              k                        ⁢                          d              k                                          )      
and the amount of alteration of the refractive index of the core and the cladding by the emission of ultraviolet light is less than 0.002 at of 674.9 nm wavelength. Here in formula 2, xcex1k is absorption coefficient in each optical fiber layer, and dk is thickness in the radius direction of each optical fiber layer.
By doing this, it becomes possible to realize a fiber type optical component in which non-uniform increase of the refractive index in the cross section of the optical fiber is small and good optical properties are obtained.
The invention according to a third aspect, in the fiber type optical component according to the first or the second aspect, is a fiber type optical component wherein the amount of change in the refractive index of the core and the cladding is less than 0.001 at of 674.9 nm wavelength.
The invention according to a fourth aspect, in the fiber type optical component according to the third aspect, is a fiber type optical component, wherein the diameter of the outermost layer on the optical fiber to which light sensitive material is applied twice as large as the diameter of mode field or more.
The invention according to a fifth aspect, in the fiber type optical component according to the fourth aspect is a fiber type optical component, wherein diameter of the outermost layer on the optical fiber to which the light sensitive material is applied 3 times as large as the diameter of the mode field or less.
By forming slant type optical fiber grating by using this optical fiber, it is possible to realize a fiber type optical component in which the filter property such that the transmission loss bandwidth is narrow and steep in reflex inhibiting angle and the transmission loss is large is obtained.
The invention according to a sixth aspect, in the fiber type optical component according to the fifth aspect, a fiber type optical component wherein the light sensitive material is germanium.
The invention according to an seventh aspect, in the fiber type optical component according to the sixth aspect, is a fiber type optical component wherein a laser which is emitted so as to enhance the refractive index of the core and the cladding is KrF eximer laser.
The invention according to a eighth aspect, in the fiber type optical component according to the seventh aspect, is a fiber type optical component wherein a periodic refractive index section is formed in the optical fiber so as to form an optical fiber grating.
The invention according to a ninth aspect, in the fiber type optical component according to the eighth aspect, is a fiber type optical component wherein a lattice vector of the grating is set to be non-zero degree towards the axis of the fiber.
As explained above, according to the present invention, by using the optical fiber which satisfies a relationship such as 24 [xcexcmxc2x7wt %] less than xcexa3kxcex2kdk less than 100 [xcexcmxc2x7wt %] of germanium density in the light sensitive layer in the core and the cladding and making the amount of the alteration of the refractive index of the core and the cladding by the emission to be less than 0.002 at 674.9 nm wavelength, it becomes possible to obtain the fiber type optical component in which nonuniform refractive index alteration is small in cross section of the optical fiber with good optical property.
Also, by using the optical fiber which satisfies relation such as 0.35 less than exp(xe2x88x922xcexa3kxcex1kdk) of the light sensitive layer in the core and the cladding and making the amount of the alteration of the refractive index of the core and the cladding by the emission of the ultraviolet light to be less than 0.002 at of 674.9 nm wavelength, it becomes possible to obtain a fiber type optical component in which nonuniform refractive index alteration is small in cross section of the optical fiber with good optical property.
Furthermore, by forming the optical fiber such that the outermost diameter of the light sensitive layer of the optical fiber is to be twice as large as the mode field diameter or more, and by forming a slant type optical fiber grating by this optical fiber, it is possible to realize a fiber type optical component in which the bandwidth of transmittance loss is narrow and steep in the reflection restricting angle with filtering ability having large transmittance loss.